DESCRIPTION (Abstract of the application) Muscle has emerged as one of the most important target tissues for gene therapy. Human clinical trial are in progress to determine the ability of intramuscularly injected plasmid DNA to induce a more effective immune response against viral infections and cancer. Muscle can also be used to manufacture and secrete proteins, such as hormones and growth factors. Genetic muscle diseases, such as Duchennes muscular dystrophy, could be treated by the expression of the respective normal gene in muscle. Finally, muscle could be used as a vehicle to clear circulating toxic metabolites that accumulate in inborn errors of metabolism. Despite this promise, a clinically-viable gene transfer method has yet to be developed. The bottleneck for realizing this treatment is the efficiency of gene transfer and stability of expression from the gene vectors developed to date. The proposed studies will address the efficiency of expression by developing new methods to deliver genes via blood vessels. Several advantages flow from the intravascular delivery of genes instead of direct intramuscular injections. Since the blood vessels are in direct and close contact with every muscle cell, delivery to the cellular surface of muscle cells can be a much more efficient process. More muscle groups could be easier reached through this method than using direct muscle injections. Breaching the endothelial barrier in muscles without toxicity is a challenging endeavor. However, many natural macromolecules and cells use different "keys" (ligands) to pass from the lumen of blood vessels into the interstitium of muscle. The development of phage display libraries provides an exciting new approach for finding the "key for unlocking this door." Phage-display libraries will be used in both in vitro and in vivo endothelial cell selection systems to identify peptides that enable transendothelial movement (TEM) of phages. These ligands will then be attached to vectors in order to enable them to transfer genes into muscle cells following their intravascular injection. Our proposed studies are likely to lead to a greater understanding of TEM in muscle and the generation of critical reagents for muscle gene therapy. These ligands will be of great use for both non-viral and viral gene transfer methods to muscle and other tissues as well.